Field of the Invention:
The present invention relates to an automatic speed reducing ratio-switching apparatus capable of automatically switching a speed reducing ratio transmitted from an output shaft, for example, to another member or apparatus through means of a planetary gear mechanism, when a load in excess of a predetermined torque is applied thereto. The present invention can be applied and utilized in a variety of extensive fields, to be described later on.
Description of the Related Art:
A speed reducing ratio-switching mechanism has been hitherto applied, for example, to machine systems such as construction machines. In such machine systems, an electric cylinder is used as an actuator in an expansion/contraction operation system, for driving a link mechanism.
In such an electric cylinder, a rotary shaft is connected to an input portion of an electric motor housed within a casing, and a screw shaft is arranged in connection with the rotary shaft. The screw shaft is screw-engaged with a nut member, which is rotatably supported inside the casing. Two pairs of planetary gear mechanisms, which have different speed reducing ratios, are provided between the rotary shaft and the nut member. Each of the planetary gear mechanisms comprises a sun gear, and planet gears, which are meshed with the sun gear and an internal gear provided inside of the cylindrical casing, so as to undergo planetary motion. The sun gears of each of the planetary gear mechanisms are connected to the rotary shaft via a one-way clutch, wherein the direction of engagement is different with respect to the forward rotation direction and the reverse rotation direction thereof. A planetary support shaft, which rotatably supports the planet gears of each of the planetary gear mechanisms, is connected to the nut member.
In the case of an electric cylinder, when the electric motor is driven and rotated in the forward rotation direction, then the rotary shaft is also rotated in the forward rotation direction, and the nut member is rotated in the forward rotation direction through the planetary gear mechanism at a small speed reducing ratio, whereby the screw shaft is operated and moved in an expandable manner. On the other hand, when the electric motor is driven and rotated in the reverse rotation direction, then the rotary shaft is also rotated in the reverse rotation direction, and the nut member is rotated in the reverse rotation direction through the planetary gear mechanism at a large speed reducing ratio, whereby the screw shaft is operated and moved in a contractible manner (see Japanese Laid-Open Patent Publication No. 2003-184982).
However, in the case of the aforementioned electric cylinder, it is necessary to use two types of planetary gear mechanisms, each having different speed reducing ratios, in order for the screw shaft to perform both an expanding operation and a contracting operation. In this case, the number of parts is increased, and the entire electric cylinder is increased in size.
In the case of the planetary gear mechanisms of the electric cylinder described above, the expanding operation is performed at a low speed and with a large thrust force, while the contracting operation is performed at a high speed with a small thrust force, irrespective of the magnitude of the load torque that is applied to the electric cylinder. Therefore, the movement speed of the screw shaft cannot be increased to a higher speed during the expanding operation, even if the load torque applied to the electric cylinder is small.
In view of the above, the present applicant has suggested an automatic speed reducing ratio-switching apparatus, which makes it possible to control torque and transmit the torque at a high speed, by automatically switching the speed reducing ratio in response to operation of a displacement member that forms an actuator (see, Japanese Patent Application No. 2005-141123).